Hereditary non-polyposis colon cancer (HNPCC) is a cancer syndrome shown to be the result of a mutator effect - inheritance of gene or genes with mutations detracting from the cells' ability to correct damage to DNA. In HNPCC, dominantly inherited defects in mismatch repair (MMR) genes lead to microsatellite instability after loss of the normal allele function in affected tissue. This is seen in tumor tissue DNA by conducting PCR across a set of microsatellite loci and observing mutant fragments at a minimum of 40% of such loci. The molecular basis of the disease in such families is arrived at by identifying which MMR gene might be responsible by immunohistochemical staining (IHC) of the tumor, and then identifying a significant mutation in that MMR gene segregating with the disease. This works fine when the mutation in the MMR gene knocks out the level of IHC-detectable enzyme after LOH and when the resultant MSI is at high enough level to be detected by simple PCR. However, MMR mutations might produce IHC detectable proteins, which are inefficient but not totally ineffectual. Such "hypomorphic" mutations would allow mutant microsatellite fragments to accumulate but at levels not detectable by standard PCR. Consistent with that hypothesis, HNPCC has been shown to segregate in a significant proportion of families whose cancers do not meet the MSI-IHC-MMR mutation criteria. The molecular basis of their genetic disease can be approached by conducting PCR at the single molecule level in many aliquots of DNA from a sample. With appropriate statistics one can quantify mutant fragments at marker microsatellite loci and generate an MSI phenotype (weighted frequency of mutant alleles over all loci) for that sample. Such small pool PCR (SP-PCR) and will be used here to determine the MSI phenotypes in both the tumor and normal (PBLs) tissues relative to the type of MMR mutation (or lack thereof) in HNPCC pedigrees. We also intend to determine the ability of the procedure to identify individuals at risk in families where no MMR mutation has been identified, and to identify "sporadic" patients carrying germline mutation predisposing to cancer. Finally, we intend to determine the impact of inherited organism-dominant/cell recessive germline mutations on future generations.